快速 (kuàisù) is a formal adjective meaning 'fast, rapid', typically used in written or technical contexts (e.g., high-speed trains, rapid development). It is less common in everyday speech than 快 (kuài), which can also be an adverb, and more technical than 迅速 (xùnsù 'swift, prompt'). Think of 快速 as 'high-speed' or 'rapid' in a measured, compound sense.